The leading compounds currently used as local spinal or epidural anesthetics are structurally related and include lidocaine, mepivacaine, prilocaine, chloroprocaine, bupivacaine, ropivacaine and levobupivacaine. Regional anesthesia using these compounds results from inhibition of the sodium ion influx of the voltage-gated sodium channel. These compounds have been shown to bind near the cytoplasmic opening of the sodium channel.
The natural products tetrodotoxin (TTX) and saxitoxin (STX) occlude the extracellular opening of the voltage-gated sodium channel at receptor site 1 (Lipkind, G. M., et al., Biophys J, 66:1-13 (1994)). Both TTX and STX are currently available only from natural sources. TTX is found in ovaries of some puffer fish species and in the eggs of the California newt. STX is produced by dinoflagellate alga species and may be found in mussels and other shellfish.
The biological activities of TTX and STX are highly selective. TTX binds to the voltage-gated sodium channel with a Kd of 3 to 5 nM (Colquhoun, D., et al., J Physiol, 221:533-553 (1972)) while concentrations 104-fold greater have been shown to have no discernible effect on other receptors (Hille, B., Ionic channels of excitable membranes, (1992)). Biologic activity thus derives solely from interaction with receptor site 1 of the voltage-gated sodium channel. Compounds which compete with TTX or STX for binding to site 1 of the voltage-gated sodium channel should therefore exhibit similar biological effects.
If administered intrathecally in the rat at a dose approximating 10×ED50 for block of the tail-flick reflex, TTX typically effects a neuronal block that lasts between two and four days. When administered intrathecally in the rat at this dose (10×ED50 for block of tail-flick reflex), TTX appears to have toxicity to neural elements indistinguishable from saline (Sakura, S., et al., Anesth Analg, 81:338-346 (1995)).
In addition to anesthetic effects, compounds affecting blockade of the voltage-gated sodium channel in general, and TTX specifically, have well documented neuroprotective properties. There is substantial evidence that potentially damaging conditions such as ischemia induce influx of sodium through TTX-sensitive channels (Fung, M. L., et al., Neurosci Lett, 275:41-44 (1999); Lysko, P. G., et al., Stroke, 25:2476-2482 (1994); Taylor, C. P., et al., Trends Pharmacol Sci, 16:309-316 (1995)). The resultant membrane depolarization impacts other voltage-sensitive mechanisms such as voltage-sensitive calcium channels, potassium channels, and glutamate release (Li, S., et al., J Neurosci, 19:RC16 (1999)). Membrane depolarization further enhances sodium entry leading to high concentrations of intracellular sodium, depletion of ATP stores, and influx of calcium via reversal of the Na+—Ca2+ exchanger. The deleterious condition need not be acute because TTX has been shown to protect motor neurons in a chronic model of glutamate toxicity developed to mimic ALS (Rothstein, J. D., et al., Proc Natl Acad Sci USA, 90:6591-6595 (1993)).
Regardless of underlying mechanism, the homeostatic, cytoprotective, and beneficial physiologic effects afforded by TTX has been demonstrated in a variety of in vitro and in vivo biological models using varied insults including vascular occlusion (Yamasaki, Y., et al., Neurosci Lett, 121:251-254 (1991)), cardiac arrest (Prenen, G. H., et al., Exp Neurol, 99:118-132 (1988)), traumatic axonal deformation (Wolf, J. A., et al., J Neurosci, 21:1923-1930 (2001)), dorsal column segment compression (Agrawal, S. K., et al., J Neurosci, 16:545-552 (1996)), anoxia (Breder, J., et al., Neuropharmacology, 39:1779-1787 (2000); Imaizumi, T., et al., J Neurotrauma, 14:299-311 (1997); Lopachin, R. M., Ann N Y Acad Sci, 890:191-203 (1999); LoPachin, R. M., et al., Neuroscience, 103:971-983 (2001); Pringle, A. K., et al., Brain Res, 755:36-46 (1997); Probert, A. W., et al., Neuropharmacology, 36:1031-1038 (1997); Stys, P. K., et al., Ann Neurol, 30:375-380 (1991); Stys, P. K., et al., J Neurosci, 12:430-439 (1992); Vornov, J. J., et al., Stroke, 25:457-464 (1994); Waxman, S. G., et al., Brain Res, 644:197-204 (1994); Weber, M. L., et al., Brain Res, 664:167-177 (1994)), glucose deprivation (Tasker, R. C., et al., J Neurosci, 12:4298-4308 (1992)) as well as excitotoxic damage induced by veratridine (Lysko, P. G., et al., Stroke, 25:2476-2482 (1994)), brevetoxins (Berman, F. W., et al., J Pharmacol Exp Ther, 290:439-444 (1999)), and NMDA receptor stimulation (Skaper, S. D., et al., J Neurochem, 76:47-55 (2001); Strijbos, P. J., et al., J Neurosci, 16:5004-5013 (1996)).
Compounds effective at blocking voltage-gated sodium channels are effective in the treatment of neuropathic pain. More specifically, TTX inhibits neuropathic ectopic activity by blockage of TTX-sensitive voltage-gated sodium channels accumulating at the site of injury (Kim, C. H., et al., Brain Res Mol Brain Res, 95:153-161 (2001); Omana-Zapata, I., et al., Pain, 72:41-49 (1997)). In-vivo administration of TTX significantly reduces allodynic behavior in a rat model (Lyu, Y. S., et al., Brain Res, 871:98-103 (2000)). Consistent with this, TTX has been used effectively to reduce neuropathic pain in patients with cancer (du Souich, P., et al., Clin. Pharmacol. Ther., 71:MPI-46 (2002)).
Voltage-gated sodium channel blockers have also been shown effective against partial and generalized tonic seizures (Catterall, W. A., Adv Neurol, 79:441-456 (1999)). More specifically, TTX has been shown to suppress seizures in rat hippocampal slices for several hours (Burack, M. A., et al., Epilepsy Res, 22:115-126 (1995)).
A compound that competes for binding to the voltage-gated sodium channel with TTX can be an anesthetic, an analgesic, a neuroprotective agent, an agent for treatment of neuropathic pain, or an anticonvulsant. Such is the case with the compounds described here.